1. Field of the Invention
The present invention relates to antennas and stripline to microstrip coupling circuits. More specifically, the present invention relates to aperture coupled stripline fed microstrip patch antennas and aperture coupled stripline to microstrip coupling circuits.
2. Description of the Related Art
Stripline and microstrip feedlines are commonly used at high operating frequencies, such as the VHF, UHF, microwave and millimeter wave frequency ranges. A stripline feedline is typically assembled from metal-clad printed circuit board substrate with two ground planes spaced apart by a dielectric substrate material. Within the dielectric material is a feedline which is formed as a flat conductive strip by etching away unwanted metal cladding. The physical dimensions of the feedline and dielectric material, as well as the dielectric constant of the dielectric material determine the impedance of the stripline feedline.
In a similar fashion, microstrip feedlines are formed from metal-clad printed circuit board substrate. A single ground plane and a feedline, spaced apart by the dielectric substrate material form the microstrip. The feedline is a flat conductive strip formed by etching away unwanted metal cladding. The impedance of the microstrip is a function of the thickness of the dielectric, its dielectric constant, and the physical dimensions of the feedline.
It is well understood by those skilled in the art that resonant structures can be formed using microstrip and stripline technology. Antennas are commonly fabricated as microstrip patches formed by etching away unwanted metal cladding, leaving behind a patch of metal cladding, the size of which is selected to be resonant at a particular frequency of operation. The patch is supported by the printed circuit board dielectric substrate over a ground plane, which is formed by the metal cladding on the opposite side of the printed circuit board.
A useful combination is to feed a microstrip patch antenna with a stripline feedline. In doing so, it is necessary to couple the signal between the antenna patch and the stripline feedline which is located between two ground planes. Drawing FIGS. 1A and 1B illustrate a prior art method of accomplishing the signal coupling.
Reference is directed to FIG. 1A which is a top view of the prior art stripline fed microstrip patch antenna. The stripline is formed with multiple metal-clad printed circuit board layers 1, which have a feedline 4 located therein. The microstrip patch antenna 2 is supported above the stripline 1 and is formed to the desired resonant characteristics. In this figure, the microstrip patch 2 is comer clipped to yield an antenna with circular polarization characteristics. The coupling of electromagnetic energy between feedline 4 to antenna patch 2 is accomplished with a coaxial feed comprising a coaxial opening 6 in the ground plane of the stripline structure 1 and a coaxial pin connector 7 which is conductively coupled to both feedline 4 and antenna 2. To prevent undesired electromagnetic propagation modes, several plated-through holes 8 are placed around the coaxial opening 6.
FIG. 1B shows a cross-section of the prior art stripline to microstrip patch antenna coupling circuit. The stripline 1 includes two dielectric substrate layers 10 and 12. At the outer edges of these two layers are ground plane surfaces 16 and 18, respectively. The feedline 4 is sandwiched between dielectric layers 10 and 12. The microstrip patch antenna 2 is insulated from the stripline 1 by dielectric layer 14. Energy is coupled from feedline 4 to antenna 2 by metal coaxial pin 6, which passes through coaxial opening 7 in ground plane layer 16. This form of coupling is known as xe2x80x9cprobe-couplingxe2x80x9d or xe2x80x9ccoaxial couplingxe2x80x9d by those skilled in the art. The plated through holes 8 conductively couple ground plane layers 16 and 18 for the purpose of suppressing undesired propagation modes.
The pin or xe2x80x98probexe2x80x99 coupling techniques work well at the lower frequency ranges since the physical dimensions are relatively large allowing generous tolerance ranges. Also, hand assembly techniques are acceptable because the physical size of the components is such that they can be hand soldered with relative ease. However, as the desired frequency of operation increases, the component sizes decease. In the Q-band, for example, frequencies in the 44 GHz range, the wavelength requires components of very small physical size. The coaxial pin would be on the order of 0.010 inches in diameter. This diameter is so small that it becomes difficult to solder to the antenna. The process then requires a very skilled technician to do the assembly work. If reflow solder techniques are used, there is an increased possibility the solder will flow so as to bridge the small insulating regions. While larger- coaxial pin sizes could be utilized, the pin becomes too close to the antenna patch size and antenna performance is degraded. Likewise, the coaxial opening may need to be so large that it becomes significant with respect to the antenna patch size.
Thus there is a need in the art for a coupling circuit design to couple high frequency signals between stripline feedline circuits and microstrip circuits, such as microstrip patch antennas, which eliminate the need for coaxial, or probe, coupling techniques.
The need in the art is addressed by the apparatus of the present invention. One embodiment of the inventive apparatus is an aperture coupled antenna, including a stripline feedline with two ground planes positioned substantially parallel to each other with dielectric material in between them. A feedline is placed within the dielectric material thus forming a stripline feedline. A resonant opening is formed in one of the ground planes and is located adjacent to an end of the feedline. A non-resonant cavity is formed with several conductors connected between the two ground planes and is located around the resonant opening. An antenna is located adjacent to the resonant opening on the opposite side of the ground plane, with the resonant opening, from the feedline. This arrangement allows electromagnetic energy to be coupled between the feedline and the antenna through the resonant opening without the need to solder a pin or probe between the feedline and the antenna.
Coupling between a stripline feedline and an antenna is not the only useful application of the present invention. It is equally useful in any situation where a stripline feedline needs to be coupled to a microstrip circuit. A second apparatus is a stripline to microstrip coupling circuit, including a stripline feedline with two ground plane positioned substantially parallel to each other with dielectric material in between them. A feedline is placed within the dielectric material thus forming a stripline feedline. A resonant opening is formed in one of the ground planes and is located adjacent to an end of the feedline. A non-resonant cavity if formed with several conductors connected between the two ground planes, and is located around the resonant opening. A stripline conductor is supported by another dielectric material on the opposite side of the ground plane, with the resonant opening, from the feedline. The stripline conductor is located adjacent to the resonant opening. This arrangement allows electromagnetic energy to be coupled between the feedline and the microstrip conductor through the resonant opening.